Film wrapping machines are used for applying a film to a load often comprising one or more articles stacked on a pallet. The wrapping machine generally comprises a stretch head with a film supply roll, wherein the stretch head and load are moved in relation to one another to supply the film from the film supply roll and wrap the film about the load. In some wrapping machines, a moving stretch head orbits a fixed load, and in other wrapping machines the load is rotated on a turntable in relation to a fixed stretch head. As the film is wrapped around the load, the stretch head applies a tension to the film that varies dependent on the relative motion between the load and stretch head, and on the shape of the load. Known stretch heads include a film roller to provide a tension on the film, wherein the film is drawn over and frictionally engages the film roller as it is applied to the load. For example, U.S. Pat. No. 5,163,264 discusses a film roller that drives a hydraulic pump which provides resistance or drag on the film roller. The pump is coupled to a hydraulic circuit with a fixed orifice pressure valve which imparts resistance to the fluid pumped through the circuit, to the pump, to the film roller, and ultimately to the film drawn over the film roller. Known film rollers, however, have the disadvantage that they provide a variable tension on the film. More specifically, the tension on the film and the corresponding force on the load increases and decreases in relation to the increasing and decreasing rate, respectively, at which the film is drawn over the film roller and applied to the load. The shape of the load, for example, corners of a square load, also affects the rate at which the film is drawn over the film roller and applied to the load. To ensure secure packaging of the load, the film must be applied at some minimum tension which may not be reached until the relative motion between the load and the stretch head reaches some threshold value after start-up of the wrapping machine. The delay required to apply sufficient tension to the film for proper wrapping, however, increases the time and material required to package the load. Similar inefficiencies result after shut-down of the film wrapping machine since the wrapping machine cannot be stopped instantaneously. Further, the rate at which the film is applied to the load is limited by the force that the load can withstand without being tipped over by the film. Also, some loads may be damaged by a high tension film which crushes the packaging comprising the load.
In the past it has been proposed to pre-stretch the film prior to applying the film to the load so as to reduce the force applied to the load. Pre-stretching the film however requires a motor driven pre-stretching apparatus generally comprising at least two rollers with different rotation rates to stretch the film. A power pre-stretching apparatus substantially increases the cost of a film wrapping machine. Further, pre-stretching the film only reduces the force on the load, and does not overcome the other problems related to inefficiency during wrapping machine start-up and shut-down, limitations on the film wrapping rate, and variations in the film tension resulting from the shape of the package. Existing methods of controlling the film tension include controlling amount of pre-stretching based on a measurement of film tension at the load by changing motor speed. These control methods however are complex and expensive, and are often not adequately responsive to changes in measured film tension as a result of the delay in feedback processing and the effect of momentum on the time required to change motor speed.
In view of the discussion above, there exists a demonstrated need for an advance in the art of a applying a constant tension to a film in a film wrapping machine.